Epidemiology of psychosomatic disorders in schizophrenic patients: methodological issues.

The value of an epidemiological study investigating the prevalence of certain somatoform diseases in schizophrenic patients can be strengthened by attention to the following methodological principles: (1) sufficient size of sample; (2) representativeness of sample, controlling if possible such variables as age range, inpatient or outpatient status, ward population or milieu and cultural aspects; (3) study of control groups in same milieu with other psychiatric diagnoses; (4) clear specification of diagnostic criteria employed; (5) establishment of interrater reliability concerning psychiatric diagnosis, patient historical data or medical record information.